Bgagger posted here about adding a quick link to invite a user to a new room, and I thought it was a great idea so here you go.
The links 'New room' & 'Invite users' in the rooms list now have small FontAwesome icons, too.

This is a very nice extension!
I see I can invite people by username. But it would be really nice to be able to invite people by group. This mass-invitation system should be guarded by the forum permissions, of course, so I imagine it won't be an easy modification.

Use case:
I have a large (8000 members) forum, with different levels of subforum access. I would like to give people with higher levels access to their communal 'private' room. Adding groups of about 300 users to a room, isn't very doable right now
It works for the moderator team-room, but even that (15 users) is some work. And when someone leaves, I have to remove them manually from the group, whereas group-permission-based would also take care of that.

Apart from minor language tweaks for the permissions, there are two new useful additions:

You can now invite groups to a room. Needless to say, there's a permission for you to control who can send invites to groups. Note that by inviting a group, you won't limit the users who can join the room to the users of this group. Other users can still join the usual ways.

The active users list has been improved: there's a new permission to control who can see the list of active users in your rooms. This allows you to stop guests from spying, for example. Note that this permission is set to No for everyone by default, so make sure to update the permission according to your needs after you have installed the update.

The main attraction in this release is a new UCP option for your users: they can select a default room which they will be put in on every page load, in addition to the Lobby. Only public rooms can be selected as a default room.

There are lots of small improvements and fixes as well. In Rooms 1.2.11 it was possible to send invites to groups the user isn't allowed to see. This has been fixed in 1.2.12. Furthermore, all Room's UCP options are now hidden if the user doesn't have permission to see rooms.

Check the change log in the first post for a complete list of changes.